Materials, such as powders, pellets, granules, fibers, and flakes, generally referred to as flowable solids, require special handling equipment for measurement and feeding. In response to this need, a number of products have been developed. The feeders and meters developed for flowable solids come in a variety of configurations. Generally, the flowable solid material is held in a hopper with a feed screw positioned at the bottom opening of the hopper. The screw turns at a selected speed and delivers the material which flows from the hopper through a discharge opening. The flow rate of the material may be measured in a number of different ways.
For example, the volumetric feeding principle can be used to measure and control the flow rate of material. Bulk material is discharged from the hopper to a feed screw so that a constant volume is fed per unit of time. The feed rate is determined through calibration: a time sample is taken and weighed and screw speed is adjusted accordingly. Feeding accuracy depends on the uniformity of the bulk material, its handling behavior, and consistent bulk density.
Alternatively, a loss-in-weight feeder principle may be used to measure and control the flow rate of material. The discharge unit, the hopper, and the product to be fed are placed on a scale or suspended in a weighing system. The total weight is stored in a computer controller memory. As bulk material is discharged by the feed screw, the weight difference per unit of time is continually measured. The measured actual value is compared with the desired value. The feeder is controlled by varying the discharge speed to make adjustments, for example, for changes in bulk density. The loss-in-weight feeder principle is the most accurate system because its computations are always based on actual weight changes.
Finally, a weight belt feeder principle can be used. Bulk material is discharged from a hopper to a driven belt across a weigh bridge. The weight acting on the weigh bridge is measured, and a computer computes the feed rate, based on weight and the belt speed. The throughput is regulated to the desired value by varying the belt speed.
Though these three different feeding principles work somewhat differently, generally, all use a hopper with a feed screw at the discharge of the hopper which moves material from the hopper and discharges the material. An example of a prior art feeder using this principle is shown in FIG. 1. Such a prior art feeder comprises a hopper 10 which holds the material to be dispensed. A pair of feed screws 14 move the material from the hopper to a discharge 16. An agitator 20 rotates to keep the material to be dispensed free-flowing and away from the sides of the hopper 10. Agitator 20 is driven by a shaft 22 which is disposed horizontally, parallel to the long axis of feed screws 14. Thus, one motor and a gear box 30 can provide power both to feed screws 14 and shaft 22. The hopper shown in FIG. 1 has a roughly semi-cylindrical cross section, which allows a horizontally driven agitator to sweep the lower portion thereof to dislodge materials. This design necessarily includes a rectangular opening 35. To this opening can be attached a larger storage bin, not shown.
A problem arises with rectangular openings when the material being fed through the opening has a tendency to bridge. In such a situation, the material can form a bridge across the short side of opening 35 (i.e., the material will pack against the long sides of the opening and form a bridge spanning the opening). When this occurs, the material being fed will cease feeding and the bridge must be broken up manually. Bridging also tends to occur near the opening leading to feed screws 14. For this reason, agitator 20 should pass as close to feed screws 14 as possible to break up any bridges.
In an alternative configuration, hopper 10 may be shaped conically, with the point of the cone downward. This provides a circular opening which reduces the tendency of a material to bridge. However in such a case, a horizontally driven agitator is not possible. Therefore, a separate motor is generally attached to the cover for the hopper, and the motor, with a vertically disposed drive shaft, drives an agitator. This is undesirable since it adds an extra drive system and therefore extra cost and complexity to the feeder. It is possible to use a mechanical arrangement to transmit power from the motor which drives the feed screws, however this too is a complex arrangement and inflexible with respect to a change in the hopper size.